1. Field of the Invention
This invention generally relates to exploration for hydrocarbons involving electrical investigations of a borehole penetrating an earth formation. More specifically, this invention relates an instrumented bit on a tool moving along the borehole within an earth formation for introducing and measuring individual survey currents injected into the wall of the borehole. Additional measurement sensors may also be provided.
2. Background of the Art
Electrical earth borehole logging is well known and various devices and various techniques have been described for this purpose. Broadly speaking, there are two categories of devices used in electrical logging devices. In the first category, called “Galvanic” devices, electrodes emit current into the earth formations and voltages are measured to give a resistivity measurement. One of the simplest forms of galvanic devices is the so-called “normal” device where a current electrodes emits a current through the earth formation to a remote return location, and a voltage electrode measures the potential due to that current with respect to the remote reference location. In the second category, inductive measuring tools, an antenna within the measuring instrument induces a current flow within the earth formation. The magnitude of the induced current is detected using either the same antenna or a separate receiver antenna. The present invention belongs to the first category.
Normal type devices have been used extensively in the logging industry to determine formations resistivity when the borehole fluid is either more resistive or is slightly less resistive than the formation. One of the earlier devices was that of Doll wherein a so-called “micronormal” tool was used for measuring resistivity near the borehole wall.
Some Galvanic devices are designed to focus the survey current into the formation whose resistivity is to be determined. For example, Birdwell (U.S. Pat. No. 3,365,658) teaches the use of a focused electrode for determination of the resistivity of subsurface formations. A survey current is emitted from a central survey electrode into adjacent earth formations. This survey current is focused into a relatively narrow beam of current outwardly from the borehole by use of a focusing current emitted from nearby focusing electrodes located adjacent the survey electrode and on either side thereof. Other Galvanic devices focus the current near the borehole wall. This is useful when the mud resistivity is much lower than the formation resistivity.
U.S. Pat. No. 4,468,623, issued to Gianzero, discloses tool mounted pads, each with a plurality of small measure electrodes from which individually measurable survey currents are injected toward the wall of the borehole. The measure electrodes are arranged in an array in which the measure electrodes are so placed at intervals along at least a circumferential direction (about the borehole axis) as to inject survey currents into the borehole wall segments which overlap with each other to a predetermined extent as the tool is moved along the borehole. The measure electrodes are made small to enable a detailed electrical investigation over a circumferentially contiguous segment of the borehole so as to obtain indications of the stratigraphy of the formation near the borehole wall as well as fractures and their orientations. In one technique, a spatially closed loop array of measure electrodes is provided around a central electrode with the array used to detect the spatial pattern of electrical energy injected by the central electrode. In another embodiment, a linear array of measure electrodes is provided to inject a flow of current into the formation over a circumferentially effectively contiguous segment of the borehole. Discrete portions of the flow of current are separably measurable so as to obtain a plurality of survey signals representative of the current density from the array and from which a detailed electrical picture of a circumferentially continuous segment of the borehole wall can be derived as the tool is moved along the borehole. In another form of an array of measure electrodes, they are arranged in a closed loop, such as a circle, to enable direct measurements of orientations of resistivity of anomalies.
The device of Gianzero '623, is primarily designed for highly conductive mud. In oil based muds, (OBM) the currents flowing from the electrodes depend upon good contact between the electrode and the borehole wall. If the borehole wall is irregular, the contact and the current from the electrodes is irregular, resulting in inaccurate imaging of the borehole. Finally, being a wireline tool, a plurality of contact pads disposed around the logging tool must be used to get a 360° scan of the borehole.
OBM increase drilling efficiency due to better lubrication of drill bits. In addition, OBM must be used when drilling through water soluble formations. An increasing number of present day exploration prospects lie beneath salt layers
U.S. Pat. No. 3,973,181 to Calvert teaches a method and apparatus for investigating earth formations traversed by the borehole containing an electrically nonconductive fluid in which a system of pad-mounted electrodes are arranged laterally around a well tool. A high frequency oscillator is coupled to the electrodes through a selectively variable capacitor. As the apparatus is passed through the borehole, the high frequency current produced by the oscillator capacitively couples the electrodes to the formation and provides a measure of the electrical conductivity or resistivity of the earth formation.
U.S. patent application Ser. No. 09/836,980 of Evans et al. having the same assignee as the present application and the contents of which are fully incorporated herein by reference, teaches an apparatus and method based on the use of high frequency measurements for injecting measure currents into the formation. One embodiment of the device and method taught in the Evans '980 application uses a carrier frequency that is sufficiently high to capacitively couple the measure electrodes to the formation through the nonconducting mud. By modulating the carrier current at a frequency used in prior art resistivity imaging devices, it is possible to get measurements of formation resistivity with relatively minor changes in the hardware of prior art resistivity devices, the main modification being a modulator for producing the measure currents demodulator for demodulation of the measure currents.
U.S. patent application Ser. No. 10/090,374, of Evans, having the same assignee as the present application and the contents of which are fully incorporated herein by reference, discloses a method for obtaining a resistivity image of an earth formation penetrated by the borehole. The apparatus includes at least one measure electrode that injects a measure current into the formation. Due to the high frequency of the current, an electrical circuit is complete when the borehole is filled with a non-conductive fluid through a capacitive gap between the electrode and the formation. A guard potential is provided to maintain focusing of the current. The modulation of the measure current and the demodulation of the output of the current measuring circuit helps reduce the cross-talk between them. Amplitude modulation reverse amplitude modulation, frequency modulation or phase modulation may be used.
Various advantages can be obtained by placing the measuring instruments close to the drill bit. The biggest advantage is to being able to know immediately and in real time the resistivity and other properties of the earth formation. This is useful in, for example, knowing the casing point the coring point. U.S. Pat. No. 5,448,227, issued to Orbin et al. discusses a measurement-while-drilling system which includes a sensor sub positioned at the lower end of a downhole motor assembly so that the sub is located near the drill bit. The sub houses instrumentalities that measure various downhole parameters such as inclination of the borehole, the natural gamma ray emission of the formations, the electrical resistivity of the formations, and a number of mechanical drilling performance parameters. Although the techniques of this patent make substantial progress in moving sensors closer to the drill bit and overcoming some of the major telemetry concerns, the sensors are still approximately 2-3 m. from the drill bit. In addition, the sensors are still located in the motor assembly and the integration of these sensors into the motor assembly can be a complicated process.
U.S. Pat. No. 6,057,784, issued to Schaaf et al. discusses an apparatus and system for making measurements at the drill bit using sensors in the bit box attached directly to the bit. Sensor measurements are transmitted via wireless telemetry to a receiver located in a conventional MWD tool. In one implementation of the Schaaf the measuring instrumentation is placed in the bit box on the drill bit. In this embodiment, the upper portion of the drill bit is a housing that contains the measuring instruments, the telemetry means and power and control devices. The drill bit housing is connected to the bit box. While moving the sensor measurements closer to the drill bit, there is still a distance between sensor devices and drill in Schaaf. In addition, since the bit box is smaller than the outer diameter of the drill, the standoffs are relatively large. Furthermore, the Schaaf device cannot be used with oil-based mud.
It would be desirable to have an apparatus and method of determination of formation resistivity that is located on or near the drill bit, is relatively insensitive to borehole rugosity and can be used with either water based or with oil-based muds. The present invention satisfies this need.